International Geologiical Congress - Oslo 2008

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OSP-07 Oceanic hypoxia: Present and past

 

Anthropogenic eutrophication causing seasonal hypoxia on the oligotrophic Israeli shelf

 

Orit Hyams-Kaphzan , Israel Oceanographic & Limnological Research, National Institute of Oceanography (Israel)
Ahuva Almogi-Labin, Geological Survey of Israel (Israel)
Barak Herut, Israel Oceanographic & Limnological Research, National Institute of Oceanography (Israel)
Chaim Benjamini, Department of Geology and Environmental Sciences, Ben Gurion University of the Negev (Israel)
 

 

Increasing awareness of the expansion of hypoxic zones in shallow water around the globe due to influx of anthropogenic nutrients calls for better understanding of these phenomena in different oceanic settings. Seasonal hypoxia develops on the Israeli Mediterranean shelf in the vicinity of an activated sewage sludge discharge point, in an otherwise oligotropic marine setting. Marine benthic biotas have been monitored at this locality for two decades. Severe O2 depletion tends to eliminate marine benthos close to the point source during late summer and fall, but seasonally, current-induced mixing and dilution of the organic material can lead to periodic sea floor aeration, and then the explosion of deposit-feeder capiteliids. To study this seasonal hypoxic cycle we focused on the role of benthic foraminifera as sensitive recorders of sea floor conditions.

Two stations at ∼36m, PL3 (eutrophic, degraded), and PL29 (oligotrophic, healthy) 5.5 km to the NE, were sampled for foraminifera and for water column and in sediment characteristics, between January 2003 and May 2004. Sediment properties vary between the two stations, with mean grain size more variable and often finer in the degraded site, particularly when current intensity declines. Chlorophyll a concentration is up to 6 times higher at PL3, reaching ∼10.6 g/g at the top sediment layer, while the total organic carbon varies to a maximum of 25-30 wt.% at PL3, compared with 0.2-1 wt.% at PL29. PL29 as well is characterized by large seasonal variation in the total standing stocks (TSS) of foraminifera, with higher numbers during late spring and summer (∼240 specimens/10cc) and lower numbers during winter (∼20 specimens/10cc), typical for an oligotrophic station. Species richness is very high with a total of 182 species with vertical distribution in the sediment as deep as 10 cm. At station PL 3, the TSS are low with hardly any seasonality (0-30/10cc), species richness is significantly lower, and depth penetration restricted mainly to the top 1 cm. In winter and spring months, sludge is redistributed by strong currents but the TSS remains low. The assemblage composition is similar in the two stations with high dominance of Ammonia tepida. At PL3, each sludge injection is dispersed in short order by winnowing during the winter and spring, with A. tepida repopulating the newly exposed sea floor immediately. At PL29, A. tepida follows a naturally seasonal cycle, reproducing mainly during summer and fall. In this manner, increasing anthropogenic nutrient load due to sewage influx at PL3 demonstrates a dramatic impact on benthic foraminifera on the oligotrophic shelf. Seasonal variability ceases to dominate, and species richness and standing stocks fall, compared to undisturbed healthy sites. The depth zone habitable by benthic foraminifera is restricted to the topmost sediment layer, and hypoxic stress causes temporary episodes of complete disappearance of live foraminiferal populations.

 

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